The present invention relates to a system and method of halftoning for multi-pass rendering, and more particularly to a system and method of halftoning for reducing the effects of inter-pass mis-registration in multi-pass printing.
With the advent of inexpensive digital color printers, methods and systems of digital halftoning have become increasingly important. It is well understood that most digital printers operate in a binary mode, i.e., for each tonal separation, a corresponding spot is either printed or not printed at a specified location or pixel. Digital halftoning controls the printing of tonal spots, where spatially averaging the printed spots of one or more tonal separations provides the illusion of the required continuous contone.
A common halftone technique is screening, which compares the required continuous tone level of each pixel for each separation with one of several predetermined threshold levels. The predetermined threshold levels are stored in a two-dimensional threshold array called a half tone screen. If, in any given region of the image, the required tone level is darker than the threshold halftone level, the pixel is turned on and printed. These pixels can be referred to as black pixels, even though they may be printed in color. Pixels not turned on are not printed and can be referred to as white pixels. It is understood in the art that the distribution of black pixels depends on the design of the halftone screen.
Some known inkjet printers print with multiple passes. With multi-pass printing, the image pixels are spatially partitioned into sets, or partitions, and a different partition is printed in each pass. Multi-pass printing can allow for better ink drying and can reduce the visibility of print head signature caused by deviations in the size and positioning of the printed dots.
A common multi-pass printer is a two-pass printer, though multi-pass printers can use any suitable number of passes. Two-pass printers print some of the black pixels in the first pass and the rest of the black pixels in the second pass. Often, one pass is printed in the forward direction of head traversal and the other in the reverse direction.
If the alignment or registration between the passes is near perfect, the graininess of the resulting images is largely unchanged in comparison with a printer that prints the entire image in a single pass. However, if there is mis-registration between the two passes the partitions can “beat” with each other to produce undesired textures that result in considerably increased graininess in the printed image.
Mis-registration in inkjet printers arises from mechanical positioning errors between the passes. Increasing the precision in the mechanical positioning can mitigate the problem of mis-registration. However, this solution can be costly due to the tight tolerances required particularly at high resolutions.
Mis-registration that is identical from page to page over the life of the printer can be detected a priori and compensated for electronically. However, electronic compensation of each individual printer also adds cost to printers. Also, electronic compensation cannot correct registration errors under half a pixel without excessive computation.
It is desirable to provide halftone screens, which reduce the effects of mis-registration in multi-pass printers in a highly accurate yet cost effective manner.